Mitochondrial dysfunction is one of the hallmarks of cancer. However, very little is known about how mitochondrial dysfunction leads to carcinogenesis. Our studies described in this proposal suggest a novel role for mitochondria in protecting cells from nuclear genome mutagenesis, an important event in human carcinogenesis. Using Saccharomyces cerevisiae as a model organism, we analyzed the consequences of disrupting mitochondrial function on mutagenesis of the nuclear genome. We measured the frequency of canavanine resistant colonies as an indicator of nuclear genome mutagenesis. Our data demonstrate that mitochondrial dysfunction leads to mutation in the nuclear genome (i) in mutant strains lacking the entire mitochondrial genome (rho[unreadable] or p[unreadable]) or those with deleted mitochondrial DNA (rho" or p") (mitochondrial genetic dysfunction, MGD) and (ii) when oxidative phosphorylation is blocked in wild type yeast by antimycin A (mitochondrial metabolic dysfunction, MMD). The nuclear mutation frequencies in both MMD and MGD cells were higher compared to untreated control and wild type cells respectively. MGD led to decreased intracellular levels of ROS. In contrast MMD led to increased intracellular levels of reactive oxygen species (ROS). We demonstrate that nuclear genome mutagenesis due to MGD is dependent on REV1, REVS or REV7 gene products, all implicated in post-replication repair (PRR). However, nuclear genome mutagenesis due to MMD does not involve REV1, REVS or REV7 genes. Furthermore, we provide evidence that Rtg2 protein (retrograde 2) involved in mitochondria-to-nucleus retrograde response pathway protect cells from nuclear genome mutagenesis. Based on these observations we hypothesize that mitochondria protect and employ multiple pathways to guard the nuclear genome against mutagenesis. We propose 4 specific aims to test the proposed hypothesis. In all cases we have preliminary data that provide the basis for the proposed specific aims. These specific aims are:1) Establish the role of mitochondria-to-nucleus retrograde response pathway in nuclear genome mutagenesis due to mitochondrial dysfunction 2) Determine whether compromised oxidative stress response leads to nuclear genome mutagenesis due to mitochondrial dysfunction 3) Determine the role of post-replication repair pathway in nuclear genome mutagenesis due to mitochondrial dysfunction 4) Determine the nature of nuclear genome mutagenesis due to mitochondrial dysfunction Our proposed study should provide insight into the molecular genetic mechanisms of nuclear genome mutagenesis due to mitochondrial dysfunction that is of fundamental significance to human cancer and other diseases.